In an effort to reduce undesirable emissions being vented from a tailpipe connected to an engine exhaust outlet, a reduction catalyst may be positioned in the exhaust passage to promote conversion of NOx into more desirable gas species including nitrogen, carbon dioxide and water. For instance, in the case of some compression ignition engines, a reductant injection nozzle is positioned in the exhaust passage upstream from the reductant catalyst in order to mix an added reductant with exhaust gases before arrival at the catalyst where a reduction reaction occurs. Some engine systems utilize a urea injection system to provide the necessary chemical elements that combine with exhaust gases and react on the surface of a reduction catalyst to convert undesirable NOx emissions into more desirable gas species prior to exiting the engine system at the tailpipe. In more sophisticated engine systems, a dedicated electronic aftertreatment controller utilizes a variety of sensors to detect the state of the exhaust flow, and respond to that sensed state with urea dosing control signals to improve equilibrium of the reduction reaction. For instance, the electronic aftertreatment controller may determine exhaust mass flow rate as well as the percentage of the NOx in the exhaust flow, and provide a urea dosing control signal to match the injected reductant mass quantity of the NOx mass flow rate to supposedly arrive at an equilibrium reduction reaction that effectively converts all of the NOx and reductant to more desirable gases leaving at the tailpipe. In one example, U.S. Patent publication 2005/0282285 teaches a method of controlling an ammonia feed rate to a selected catalytic reduction reactor using a NOx sensor cross sensitive to ammonia.
While steady state reductant dosing control has proven somewhat effective, those skilled in the art appreciate that engine operating states are often changing, resulting in quick changes in exhaust mass flow rate, the percentage of NOx in the exhaust flow, and maybe more importantly temperature fluctuations in the exhaust flow. Because the reduction reaction is generally sensitive to temperature, the quality of the reduction reaction can change rapidly, sometimes resulting in an outgas slip event. If excess reductant or urea is present, a so called ammonia slip event might occur. On the otherhand, if insufficient reductant is present or the reduction reaction has been repressed, a NOx slip event might occur. Out gas slip events are undesirable and generally cannot be undone after having occurred.
The present disclosure is directed to one or more of the problems set forth above.